What does a Cochlear Implant do?
CIs are most often used when an individual has sensorineural hearing loss, which is typically caused by damage to the hair cells in the cochlea (the transducers of sound to the nervous system). CIs partially restore hearing by circumventing these non-functioning inner hair cells and directly exciting the auditory nerve.
How does a CI work?
Normally, the inner ear or cochlea divides the auditory signal into its different pitches or frequencies (think of the range of notes on a piano, organized from low to high). CIs process speech by:
- filtering the signal into a set of separate frequency information channels
- extracting the temporal envelope (or overall patterns of amplitude from each channel)
- having those temporal envelopes modulate electrical pulse trains generated by electrodes in the inner ear.
What does this do to the sound?
This results in major changes to the CI signal compared to the natural speech signal, including;
- the spectral resolution of the CI is far poorer than of the typical auditory system; this results in "blurred" hearing, or smearing of important speech information like the vowel formants and consonant transitions
- temporal fine structure (or the rapid changes in the waveform typically associated with pitch and periodicity) is removed, which greatly reduces the salience of pitch cues (making it harder to her music, identify a particular talker, distinguish statements from questions, etc.
Despite these changes, speech through a CI remains high intelligible because the speech signal contains many redundant cues. As such, speech can undergo massive distortions and still be highly intelligible. Yet listeners vary in the degree to which they can interpret this reduced signal, which helps explains why people vary in the level of success they show with a CI.